US3567939A - Method and apparatus for mitigating surface disruption of x-ray tube targets - Google Patents

Method and apparatus for mitigating surface disruption of x-ray tube targets Download PDF

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Publication number
US3567939A
US3567939A US772926A US3567939DA US3567939A US 3567939 A US3567939 A US 3567939A US 772926 A US772926 A US 772926A US 3567939D A US3567939D A US 3567939DA US 3567939 A US3567939 A US 3567939A
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target
ray
voltage
exposure
temperature
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US772926A
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Harvey W Schadler
William P Holland
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General Electric Co
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General Electric Co
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05G—X-RAY TECHNIQUE
    • H05G1/00—X-ray apparatus involving X-ray tubes; Circuits therefor
    • H05G1/08—Electrical details
    • H05G1/26—Measuring, controlling or protecting
    • H05G1/30—Controlling
    • H05G1/36—Temperature of anode; Brightness of image power

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  • Disruption of the target surface results in a marked decrease in the useful radiation from the tube because the probability of an x-ray photon escaping from the target is less for a rough surface than for a smooth surface.
  • operators of xray apparatus tend to compensate for decreased radiation output by increasing the power level of the electron beam. High power densities cause still higher thermal gradients that create greater mechanical stresses which in turn cause more fractures to initiate and propagate in the surface of the target. This cyclic process leads to rapid failure of the tube.
  • Metallurgists have attempted to solve the surface fracturing problem in high temperature application of metals by a variety of means including increasing the purity of the metal, decreasing the grain size, redistributing impurities, controlling grain orientation, cold working and alloying. All of these techniques tend to lower the nil ductility temperature (NDT) above which metals are increasingly ductile and below which they are brittle.
  • NDT nil ductility temperature
  • FIG. 1 shows a typical plot of ductility versus temperature of a metal. Below temperature A the metal is brittle; this temperature region is called the brittle range. Above temperature C the metal is quite ductile; this temperature region is called the ductile range. Between temperatures A and C there is a region in which the ductility of the metal increases rapidly; this temperature region is called the transition range.
  • Nil ductility temperature can be defined as that temperature at which a linear extrapolation of the linear, high slope portion of the ductility-temperature curve intersects the temperature axis at the point which is marked B.
  • ductility may be expressed in several ways. In the curve shown, ductility may be considered as being expressed in terms of percent reduction in cross-sectional area at the fracture interface when a test piece is subjected to tensile stress and elongated until fracture occurs. Greater ductility is then indicated by greater reduction of area. Ductility may be expressed in terms of bending angle if a bending test is used.
  • the NDT or point B would shift along the temperature axis, but would nevertheless, be a rather definite temperature regardless of the magnitude of the curve.
  • a metal When a metal is ductile and is subjected to stress causing thermal gradients at its surface, the stresses are relieved by plastic deformation rather than by brittle fracture. When the metal deforms plastically, the stresses are relieved before brittle fracture can occur. In general, the tendency for brittle fracture is reduced by lowering the NDT of a metal, because the metal exists in a brittle state for a shorter period during a given thermal cycle.
  • x-ray tube targets are usually made of high atomic weight refractory metals such as molybdenum and tungsten. These metals are particularly susceptible to brittle fracture because they have relatively high NDTs and broad transition ranges as conventionally processed. Moreover, in the course of normal usage, the grain structure of a tungsten xray tube target may be subject to grain growth, thereby increasing the NDT even further.
  • high atomic weight refractory metals such as molybdenum and tungsten.
  • An object of the present invention is to mitigate surface disruption of x-ray tube targets and thereby maintain their output of x-radiation at a relatively constant level.
  • This object is achieved by providing x-ray apparatus with means for maintaining the steady-state temperature of the target at a predetermined level at least above the NDT of the target material during periods of rapid heating or cooling of the target surface layer.
  • the target surface is maintained in a more ductile state during periods of high mechanical stress resulting from differential thermal expansion in the target.
  • Ductility of the target surface region insures that it will react to stress by plastic deformation rather than by brittle fracture. The prevention of brittle fracture greatly reduces disruption of the target surface.
  • the steady-state temperature of the target is maintained above its NDT and preferably fairly high in the transition range by heating the target with an electron beam of medium energy before and after high energy x-ray exposures are made.
  • a preferred approach is to utilize an existing filament in the x-ray tube as the source of the medium energy electrons for auxiliary heating.
  • the x-ray apparatus is provided with a separate low voltage source that can be connected to the x-ray tube by means of an automatic switch when no exposure is being made.
  • the x-ray tube filament current is automatically adjusted so that the total power density of the electron beam is small as compared with the power density during sustained radiographic exposures.
  • This heating power is sufficient to maintain the temperature of targets made of any the tube the common refractory metals stress their NDT.
  • the auxiliary heating is discontinued and is not restarted until the target temperature falls to a predetermined level after some of the excess heat that usually results from an exposure is dissipated.
  • An additional object and benefit of maintaining the target above its NDT is that the tube is kept warm at all times. This facilitates distribution of electric charge within the tube and relieves the electrostatic stress when high voltage is applied in which case the likelihood of puncturing the glass envelope of the tube is reduced.
  • Another object and benefit of keeping the target warm is that the bearing structures of rotating anode tubes are subjected to less severe temperature variations than is the case with a cold tube when an ordinary high energy exposure is initiated. As a result, the bearings of the tube run more quietly and with less vibration.
  • FIG. 1 is a plot of ductility versus temperature for a representative metal
  • FIG. 2 is a schematic representation of the principal components of a conventional x-ray tube power supply circuit in which the invention has been incorporated.
  • a conventional rotating target x-ray tube is designated by the reference numeral 1.
  • the tube includes a directly heated cathode or filament 2 in spaced relationship with a rotating anode or target 3.
  • Most tubes have several cathodes which are used alternatively for radiography and fluoroscopy, and for the purposes of the invention, an additional cathode may be used. All or some of the cathodes may be indirectly heated.
  • the one filament shown in the tube of this example is heated by current from the secondary winding of a filament transformer 4.
  • the primary of transformer 4 is energized through a common line 5 on one side of the primary and alternative lines 6 and 7 on the other side, Different voltages are applied to transformer 4 from an x-ray tube current control, not shown, which is conventional and is included in the x-ray control and power supply device 8.
  • the filaments 2 or cathodes are usually energized with low voltage to keep them at standby temperature when the x-ray apparatus is turned on and they are automatically switched to higher voltages and full electron emissivity just before an exposure is made. Such last-mentioned switching means are not shown.
  • Lines 6 and 7 include two sets of relay contacts which are designated generally by the reference numeral 9.
  • one contact is closed as shown, in order to cause filament 2 to heat to a certain comparatively high temperature and emit electrons that are focused and impinged on target 3 to generate x-rays.
  • the electron beam current from filament 2 to target 3 may range from a few milliamperes of long duration when making a fluoroscopic study to a thousand or more milliamperes for a fraction of a second when making a radiograph.
  • Contactor 9 is operated by a coil 10 which closes the normally open contact in series with line 7 automatically prior to and between x-ray exposures. When this happens, a comparatively lower voltage is applied to filament transformer 4 and there is a corresponding reduction of filament temperature which results in low electron beam current through the x-ray tube 1.
  • High voltage is applied across the filament terminal 11 and the anode terminal 12 of the x-ray tube from a full wave bridge rectifier 13.
  • the AC input lines 14 and 15 for rectifier 13 extend from the secondary winding of a high voltage x-ray transformer 16.
  • the voltage supplied to the primary of transformer 16 is adjustable at the will of the operator by such means as, for example, selecting autotransformer taps, not shown, in the x-ray control and power supply 8.
  • the number of primary voltage steps available are usually such that high voltages ranging from about peak kilovolts to 150 peak kilovolts can be produced on the secondary of transformer 16,
  • Relays 17 and 18 are in the high voltage circuit.
  • the lower contacts of each relay are shown closed as they would be during a normal xray exposure.
  • a circuit is completed from DC terminal 19 of rectifier 13 through closed contact 18, to x-ray tube 1, and through closed contact 17 and back to the other DC rectifier terminal 20.
  • the contacts of contactors 17 and 18 are transferred by operation of relay 10 to close their respective pairs of contacts and connects a low voltage DC power source 21 across terminals 11 and 12 of the x-ray tube. Tracing the circuit will demonstrate that the low voltage DC source 21 is isolated from the high voltage supply described above when contactors 117 and 18 are transferred.
  • the low voltage DC source 21 is preferably limited to 10 kilovolts peak voltage. Some low intensity x-radiation will be generated in x-ray tube 1 when the low voltage from source 21 is applied to the tube, but this soft radiation can be absorbed by the x-ray tube envelope. Of course, the absorption by tubes of different manufactures will vary so it is desirable to run carefully controlled tests to determine the maximum voltage that can be applied without causing excessive x-ray output.
  • the low voltage supply should be designed so that it cannot be adjusted to a voltage that will cause an undesired amount of xray output.
  • a temperature sensing device 22 which may be a semiconductor, is placed near the x-ray tube 1 in order to sense the temperature of the x-ray tube target 3 optically.
  • This device 22 is connected to the x-ray control 8 by conductors 23 and 24.
  • the function of device 22 and the logic circuitry, not shown, in which it is incorporated is to turn on auxiliary heating or preheating if the target 3 is below its NDT or other predetermined temperature in the transition range, and to turn it off if the target exceeds the predetermined temperature from heat due to auxiliary heating or to making an exposure.
  • the x-ray control is interlocked so that relay coil 10 is energized when power comes on. This causes all the contacts which are shown open in the drawing to close in which case low voltage from source 21 and low electron beam current are applied to the x-ray tube to heat the target and bring it up to at least above its NDT. For the heaviest targets that are now in use, it has been found that about 5 minutes are required for initially preheating the target to a satisfactory level.
  • relay coil 10 When a normal x-ray exposure is initiated, relay coil 10 is deenergized and the contacts in contactors 9, l7, and 18 are transferred to the position in which they are shown, in which case the voltage and current through the x-ray tube 1 are subject to the control of the conventional current and voltage control device which are found in the x-ray control and power supply 8.
  • the temperature of the target 3 as determined by temperature sensor 22 is compared to the desired steady-state target temperature T, which is a temperature in the ductile range of the particular tube target. If the target temperature is above T, relay 10 is not reenergized and normal target cooling proceeds until the target temperature falls below T at which time relay 10 is reenergized. If at the termination of the x-ray exposures, the target temperature is below T, relay 10 is energized and electron bombardment heating of the target 3 resumes.
  • the T that is chosen will usually be nearer to temperature C in the transition range of FIG. 1 than to temperature B so that the target will be near its highly ductile state when subjected to normal exposure loading.
  • the target temperature sensor 22 and its associated circuitry to control 8 act to maintain the target 3 at a temperature as close to T as possible. If T is exceeded either during auxiliary heating or during normal x-ray exposures, relay 10 is deenergized and the power from low voltage supply 21 is prevented from reaching tube 1. Alternatively, if the target temperature falls below T, relay 10 is energized, permitting power from supply 21 to reach the tube.
  • the wattage required for maintaining x-ray tube target 3 above its NDT and in or slightly above its transition temperature range depends on a number of factors including the heat dissipation of the tube, the mass of the target, and the type of material out of which the target is made. Molybdenum targets may have an NDT below C, Tungsten targets may have an NDT ranging from 200 to greater than 600 C., depending on their structure and metallurgical history, It is desirable not to exceed the transition temperature range of a particular material by an undue margin of safety because some of the thermal capacity of the tube is utilized by auxiliary heating or preheating the target.
  • auxiliary heating power for x-ray tubes that have a massive tungsten target about 3 inches in diameter and nominally onefourth to one-half inch thick.
  • This heat is generated with a low voltage DC power supply 21 operating at 9 kilovolts peak and with the filament heat adjusted so that a little more than 20 milliamperes are conducted by the x-ray tube under standby conditions.
  • a low voltage DC power supply 21 which is adjustable between 2 and kilovolts peak will be satisfactory for almost any rotating anode x-ray tube that is currently in use.
  • 100 to 500 watts of power appear'sto offer a sufiiciently broad range to handle almost any x-ray tube.
  • ion bombardment may be the most practical way of heating. Electrical resistance heating, infrared heating, laser heating, radiofrequency and ultrasonic heating may also be used.
  • a suitable control voltage may be applied to the grid between exposures in order to limit electron beam current in the tube during auxiliary heating. If it is desired to provide auxiliary heating with low electron beam currents and a voltage that is so high as to cause undue x-ray emission from the tube, an automatic shutter, not shown, may be provided over the xray tube exit window to provide sufficient shielding during the time intervals in which auxiliary heating is activated.
  • a simple and effective method of reducing xray tube target surface disruption involves establishing the target at above its nil ductility temperature, at least, and preferably high in the transition range, before normal xray exposures are made.
  • This selective auxiliary heating is achieved in the illustrative embodiment with electron heating by operating the x-ray tube at a relatively low power level before normal high power exposures are made.
  • Other methods for providing auxiliary heating of the target are suggested.
  • X-ray apparatus that is adapted to mitigate brittle fracture of the target of an x-ray tube which is used in the apparatus by maintaining the target surface material above its nil ductility temperature prior to making an x-ray exposure, comprising:
  • terminals for receiving an x-ray tube therebetween and for applying a voltage between a cathode and a target of the tube;
  • an x-ray power supply adapted to selectively energize the tube for making an x-ray exposure an incident of which is the development of high thermal gradients and stresses in the target;
  • said auxiliary heating means comprises a source of voltage
  • switch means said switch means being operated in response to said target temperature sensing means and being adapted to selectively apply said voltage between the target terminal and a cathode terminal of an x-ray tube priorto an exposure.
  • the peak volt age limit of said voltage source before an exposure is a voltage at which no significant x-radiation is emitted from the x-ray tube during auxiliary heating.
  • X-ray apparatus that is adapted to mitigate brittle fracture of the target of an x-ray tube which is used in the apparatus by maintaining the target surface material above its nil ductility temperature prior to making an x-ray exposure, com prising:
  • terminals for receiving an x-ray tube therebetween and for applying a voltage between acathode and a target of the tube;
  • a switch means adapted to connect alternatively one or the other of the voltage sources across the target and a cathode

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US772926A 1968-11-04 1968-11-04 Method and apparatus for mitigating surface disruption of x-ray tube targets Expired - Lifetime US3567939A (en)

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4631742A (en) * 1985-02-25 1986-12-23 General Electric Company Electronic control of rotating anode microfocus x-ray tubes for anode life extension
WO2004047504A1 (de) 2002-11-21 2004-06-03 Heuft Systemtechnik Gmbh Röntgenanlage zur erzeugung von kurzen röntgenstrahlenimpulsen und mit einer solchen röntgenanlage arbeitende inspektionsvorrichtung
US20100142680A1 (en) * 2008-12-09 2010-06-10 Ryan Paul August System and method to maintain target material in ductile state
US20130027832A1 (en) * 2011-07-27 2013-01-31 The Watt Stopper, Inc. Method and Apparatus for Isolating High Voltage Power Control Elements
DE102013203218A1 (de) * 2013-02-27 2014-08-28 Siemens Aktiengesellschaft Verfahren zum Betreiben einer Vorrichtung sowie Vorrichtung
EP3777692A1 (de) * 2019-08-16 2021-02-17 GE Precision Healthcare LLC Verfahren und systeme zur konditionierung von röntgenröhren

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4631742A (en) * 1985-02-25 1986-12-23 General Electric Company Electronic control of rotating anode microfocus x-ray tubes for anode life extension
CN1998271B (zh) * 2002-11-21 2011-02-02 霍伊夫特系统技术有限公司 生成短时x射线脉冲的x射线单元以及使用该x射线单元的检验装置
KR101026313B1 (ko) 2002-11-21 2011-03-31 호이프트 시스템테크니크 게엠베하 짧은 ｘ―레이 펄스를 생성하는 ｘ―레이 장치, 및 이러한ｘ―레이 장치에 의하여 동작하는 검사 장치
US7079623B2 (en) * 2002-11-21 2006-07-18 Heuft Systemtechnik Gmbh X-ray unit for the generation of brief X-ray pulses and inspection device operating with such an X-ray unit
RU2328838C2 (ru) * 2002-11-21 2008-07-10 Хойфт Зюстемтехник Гмбх Рентгеновская установка для формирования коротких импульсов рентгеновских лучей и устройство проверки, работающее с такой рентгеновской установкой
US20060013363A1 (en) * 2002-11-21 2006-01-19 Heuft Systemtechnik Gmbh X-ray apparatus for generating short x-ray pulses, and inspecting device operating by means of such an x-ray apparatus
WO2004047504A1 (de) 2002-11-21 2004-06-03 Heuft Systemtechnik Gmbh Röntgenanlage zur erzeugung von kurzen röntgenstrahlenimpulsen und mit einer solchen röntgenanlage arbeitende inspektionsvorrichtung
US7974383B2 (en) * 2008-12-09 2011-07-05 General Electric Company System and method to maintain target material in ductile state
US20100142680A1 (en) * 2008-12-09 2010-06-10 Ryan Paul August System and method to maintain target material in ductile state
US20130027832A1 (en) * 2011-07-27 2013-01-31 The Watt Stopper, Inc. Method and Apparatus for Isolating High Voltage Power Control Elements
US8638539B2 (en) * 2011-07-27 2014-01-28 The Watt Stopper, Inc. Method and apparatus for isolating high voltage power control elements
DE102013203218A1 (de) * 2013-02-27 2014-08-28 Siemens Aktiengesellschaft Verfahren zum Betreiben einer Vorrichtung sowie Vorrichtung
EP3777692A1 (de) * 2019-08-16 2021-02-17 GE Precision Healthcare LLC Verfahren und systeme zur konditionierung von röntgenröhren
US11147528B2 (en) * 2019-08-16 2021-10-19 GE Precision Healthcare LLC Methods and systems for X-ray tube conditioning
US11712216B2 (en) 2019-08-16 2023-08-01 GE Precision Healthcare LLC Methods and systems for x-ray tube conditioning

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NL6916628A (de) 1970-05-08
DE1952859A1 (de) 1970-05-06

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